This invention relates to improvements in tow bars for product conveyor systems and, in particular, to a shock-absorbing tow bar that utilizes a bidirectional friction brake to couple a load-supporting carrier to a powered component of the movable conveyor.
As a product travels through a conveyor path, there are stations at which the product and carrier must halt and later resume travel. Stopping a load travelling at full conveyor speed, or restarting a load by engaging the carrier with a power source operating at full conveyor speed, can cause significant impact forces or shocks to be transmitted to both the conveyor apparatus and the product itself. These impacts can cause excessive noise in the work environment and damage to the conveyor and product. The load may shift or become dislodged causing an unsafe work environment. Shocks to the conveyor and product can cause attachment devices to fail and increase faults such as stress fractures in the product.
Industrial conveyor systems, including those of the power and free type disclosed herein, typically utilize tow bars between the powered component of the moving conveyor and one or more trailing, load-supporting carriers. Referring particularly to power and free conveyor systems, the powered component is the accumulating trolley on the free track and, when driven, is engaged by a pusher dog projecting from the conveyor chain on the power track. The accumulating trolley is the lead trolley and is connected to a trailing load trolley (or trollies) with a tow bar. Due to the rigidity of the trolley train and carrier assembly, the impact of a pusher dog engaging the accumulating trolley, or the impact of the accumulating trolley striking a stop, is imparted directly to the carrier under tow and may cause the load to shift, damage to the product, or excessive fatigue and wear on the components of the conveyor system.
To alleviate this excessive shock loading, a shock-absorbing link between the driven and towed components of industrial conveyor systems is highly desirable in order to provide a means of controlling the rapid acceleration and deceleration inherent in normal operation of the systems. One such device is an air-type shock absorber utilizing a piston that operates in a pneumatic chamber, an orifice through the piston permitting movement thereof only at a controlled rate. Also, similar devices have been employed of the hydraulic type and have the advantage of improved control due to the incompressibility of hydraulic fluid. An example of the air-type shock absorber is shown and described in U.S. Pat. No. 3,720,172 to Clarence A. Dehne, issued Mar. 13, 1973.
Furthermore, as the hydraulic-type shock absorber is subject to eventual leakage problems which render it totally inoperable and can cause contamination of the plant area occupied by the conveyor, a shock absorber utilizing metallic balls has been employed in an attempt to avoid the disadvantages of air and hydraulic-type shock absorbers. Such a metallic ball device is disclosed in U.S. Pat. No. 5,027,715 to Archie S. Moore et al, issued Jul. 2, 1991 where particulate damping material such as a quantity of ball bearings is positioned in a damping chamber. Acceleration and deceleration cause the bearings to be drawn past a piston through an annular space between the piston and the surrounding wall of the damping chamber. As the bearings become crowded on one side of the piston or the other, the resistance to movement increases. A disadvantage, however, is that over a period of time the piston abrades the surfaces of the balls and can cause them to fracture, thus their ability to roll lessens and the shock absorbing ability is degraded.
More recently, a shock absorbing tow bar has been developed and utilized in power and free conveyors and is disclosed in U.S. Pat. No. 5,511,486 to Pollard et al, issued Apr. 30, 1996, owned by the assignee hereof. A dampener tube is employed in which an oversized plunger moves against the resistance of a sleeve of resilient material thereby causing compression and displacement of the material as the plunger head shifts in response to rapid acceleration or deceleration of the conveyor. Although successful, the useful life of the tow bar can be a limitation along with its inability to withstand harsh operating conditions such as elevated heat and exposure to paint and paint solvents.
It is, therefore, the primary object of the present invention to provide a tow bar for a product conveyor which controls acceleration and deceleration and absorbs the shock that would otherwise be applied to the conveyor and the product, but accomplishes these results without the use of compressed air, hydraulic fluid, resilient materials or parts requiring close machining tolerances.
As a corollary to the foregoing object, it is an important aim of this invention to provide a tow bar for a conveyor in which relatively movable friction elements in a sandwich configuration respond to rapid acceleration and deceleration and absorb the shock by sliding engagement at internal wear surfaces.
Another important object is provide a tow bar as aforesaid in which the two friction elements comprise a rigid tongue slidably sandwiched between opposing wear surfaces.
Another important object is to provide a tow bar as aforesaid wherein pressure exerted by the wear surfaces on the tongue is adjustable to increase or decrease resistance to said relative movement.
Another important object is to provide a tow bar that can operate in a dynamic environment where it is repetitively subjected to high axial forces in response to acceleration and deceleration of associated conveyor components.
Another important object is to provide a tow bar for conveyors having an extended life and which can operate in a dynamic environment to dampen or absorb repetitive impacts.
Another important object is to provide a tow bar for conveyors that can provide shock absorption through multiple consecutive compressing impacts or multiple consecutive extending impacts.
Still another important objective is to provide such a tow bar that is able to withstand harsh operating conditions such as elevated heat and exposure to paint and paint solvents.
Another important object of the invention is to provide a tow bar construction of this type having an outer, protective sleeve which shields the friction elements from contaminants and enhances the structural integrity of the tow bar assembly.
Yet another important object of the invention is to provide a tow bar for conveyors of sufficient durability to undergo repetitive compression and extension hundreds of thousands of times during the life of the tow bar, and which is fail-safe in the event of a failure in response to an extending shock.
Other objects and advantages of this invention will become apparent from the following description taken in connection with the accompanying drawings, wherein is set forth by way of illustration and example, a now preferred embodiment of this invention.